1. Field of the Invention
The present invention generally relates to antennas, and more specifically to a semi-coaxial horn antenna.
2. Related Art
The radio transmission of both analog and digital information has normally been effected by one of two methods. In one, referred to as an amplitude modulation, the amplitude of a continuous sinusoidal carrier signal is modulated according to an information signal. When the amplitude modulated signal is received at a receiving location, the reverse process (that is, demodulation of the carrier) is preformed to recover the information signal. The other method employs what is termed frequency modulation. In frequency modulation, the frequency of the carrier signal is modulated in accordance with the information signal. During demodulation, circuitry is employed that performs frequency discrimination, which converts frequency variations in the carrier signal to an amplitude output that is in accordance with the original modulation. In both systems, the continuous sinusoidal carrier occupies a distinctive frequency bandwidth, or channel. This channel occupies spectrum space during transmission, which should not be utilized for other signal transmissions if interference is to be avoided.
In today""s communications environment, frequency spectrum is in short supply and is very crowded. According, there is a tremendous need for some method of expanding the availability of frequency spectrum for increased communications. In furtherance thereof, new methods and systems of communications have been developed that employ a wider frequency spectrum, rather than discrete frequency channels, for radio communications links. More specifically, new methods and systems of communications have been developed that utilize wide band or ultra wide band (UWB) technology, which is also called impulse radio communications herein.
Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation (also referred to as digital time shift modulation). Pulse position modulation is a form of time modulation where the value of each instantaneous sample of a modulating signal varies the position of a pulse in time. More specifically, in pulse position modulation, the pulse-to-pulse interval is typically varied on a pulse-by-pulse basis by two components: a pseudo-random code component and an information component. That is, when coding is used each pulse is shifted by a coding amount, and information modulation is accomplished by shifting the coded time position by an additional amount (that is, in addition to PN code dither) in response to an information signal. This additional amount (that is, the information modulation dither) is typically very small relative to the PN code shift. For example, in a 10 mega pulse per second (Mpps) system with a center frequency of 2 GHz, the PN code may command pulse position variations over a range of 100 nsec; whereas, the information modulation may only deviate the pulse position by 150 ps (which is typically less then the wavelength of a pulse).
The impulse radio receiver is a homodyne receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The baseband signal is the basic information channel for the basic impulse radio communications system, and is also referred to as the information bandwidth. The data rate of the impulse radio transmission is only a fraction of the periodic timing signal used as a time base. Each data bit time position modulates many pulses of the periodic timing signal. This yields a modulated, coded timing signal that comprises a train of identical pulses for each single data bit. The cross correlator of the impulse radio receiver integrates multiple pulses to recover the transmitted information.
Ultra wide-band (UWB) communications systems, such as impulse radio, pose very substantial requirements on antennas. Many antennas are highly resonant and have operating bandwidths of only a few percent. Therefore, when a substantial frequency change is made, then it becomes necessary to select a different antenna that has different dimensions. Such xe2x80x9ctuned,xe2x80x9d narrow bandwidth antennas may be entirely satisfactory or even desirable for single frequency or narrow band applications. In many situations, however, wider bandwidths may be required.
Conventional UWB antennas with wide bandwidths are physically large antennas, and thus impractical for most common uses. In other words, the physical size of conventional UWB antennas can be a problem in applications that require a portable system that utilizes a conventional UWB antenna.
For these reasons, many in the ultra wide-band communications environment have recognized a need for an improved antenna having wide-band performance in a sufficiently small package size that is portable.
The present invention is directed to an ultra wide-band semi-coaxial horn (SCH) antenna that is capable of transmitting and receiving impulse radio waves. The SCH antenna includes an inner conductor that is surrounded by a substantially cone-shaped dielectric, and an outer conductor that is conformally attached to the cone-shaped dielectric. The inner conductor has a substantially circular cross-section, and the outer conductor has substantially arc-shaped cross-section. The cross-section of the outer conductor defines a sector angle xcfx86 that varies along a length of the antenna to provide a desired impedance profile from an input of the antenna to an output of the antenna. In embodiments, the impedance profile is substantially constant along the length of the SCH antenna, thereby minimizing VSWR when matched to a system impedance. Alternatively, various non-constant impedance profiles can be implemented.
In embodiments, the outer conductor is the radiating conductor, and the inner conductor provides a ground for the radiating conductor. Alternatively, the roles can be reversed so that the inner conductor is the radiating conductor and the outer conductor provides a ground and a backplane for the inner conductor.
In embodiments, the inner conductor is a hollow cylinder that is able to slide over a separate shaft. The separate shaft can be part of a handtool device. Alternatively, the inner conductor can be a solid cylinder.
In embodiments, the SCH antenna includes two (or more) outer conductors, at least one for transmit operations and one for receive operations.
In embodiments, the SCH antenna is the antenna for an object finder system that detects objects behind a wall, a barrier, or other medium. In these embodiments, the dielectric of the SCH antenna can be selected to substantially match the dielectric of the target medium.
In embodiments, the dielectric is extended beyond the outer conductor so as to form a dielectric lens at the output of the semi-coaxial horn antenna.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.